How to Inspect Engine Belts and the Tensioner on Sterndrives, Inboards and Diesels

Engine off and cool, key out, battery switch off. Open the engine hatch and give yourself good light.

Take a clear photo of the belt path around every pulley before you touch anything. Many engines have a routing decal, but it may be faded or missing. You will need this if the belt comes off.

Run a flashlight along the whole belt, turning the engine by hand only if your manual says that is safe. BoatUS says to look for cracks on the top and the inside of the belt and fraying at the edges, and to replace a belt that shows them. Also look for:

• Shiny, glazed sides or ribs (the belt has been slipping)
• Chunks missing from the ribs
• Oil, coolant or fuel soaking the belt
• A belt riding off-center on a pulley

A lot of fine black dust around the belt path is a warning. Practical Sailor says heavy black belt dust means the belt is too loose, the pulleys are misaligned, or the belt is the wrong type. BoatUS adds that some dust is normal after long running. Fix any oil or coolant leak dripping on the belt, or the new belt will not last either.

BoatUS's thumb test: with the engine off and cool, press on the middle of the longest run between two pulleys. Typically the belt should move no more than about half an inch, but check your manual for where to press and how far it should move. A belt tension gauge takes the guesswork out; Practical Sailor recommends a small, inexpensive pocket gauge for every boat owner's toolbox.

Practical Sailor found most belts it checked were too loose rather than too tight. A loose belt slips, cuts alternator output and lets the alternator run hot; a belt that is too tight wears out pump and alternator bearings.

On spring-tensioned serpentine belts, do not judge tension by pushing on the belt; look at the tensioner instead:

• Many tensioners have marks showing the acceptable range. If the pointer is past the "replace" mark, the belt has stretched too far.
• Rust streaks, a cracked arm, or an arm that sits crooked are bad signs.
• With the belt off (step 8), use the proper tool to move the arm through its travel. It should move smoothly and spring back firmly, with no grinding or sticking.

A tensioner that is weak or seized lets the belt slip or jump off, as in the BoatUS story.

With the belt off, spin each idler and tensioner pulley by hand. It should turn smoothly and quietly with no wobble. A pulley that grinds, rattles, wobbles or feels rough has a failing bearing; replace it. Also look for cracked pulley edges or rust in the grooves. BoatUS says pulley misalignment is checked with a straight edge across the grooves with the belt off, and suggests leaving that to a qualified mechanic.

• Put the tool or breaker bar on the tensioner and move it the way your manual shows to release the belt. Slip the belt off one smooth pulley, then let the tensioner back slowly.
• Compare the new belt with the old one: same length, width and number of ribs. Mercury's FAQ lists belts among the spare parts to carry for a MerCruiser, so get the exact part.
• Route the new belt using your photo, release the tensioner onto it gently, and check that every rib sits in its groove on every pulley.

BoatUS describes the usual method:

• Loosen the alternator's pivot bolt slightly (it must still let the alternator swing), then loosen the bolt in the slotted adjusting arm.
• Swap the belt if needed.
• Pry gently to tension the belt, pressing on the alternator's mounting bracket or a solid part of its casing, never on a part that could crack. A wooden hammer handle works well.
• Hold the tension, tighten the bolt in the slot, then the pivot bolt. A helper makes this much easier.
• Recheck the deflection and adjust until it matches your manual.

Run the engine only with cooling water: the boat in the water with the seacock open, or water supplied to the drive or intake the way your manual allows. Stay clear of the belts. Watch the temperature gauge and the voltmeter (charging voltage should rise above resting battery voltage). Listen for squeal. BoatUS suggests testing with an electrical load on, and warns that a new belt loosens somewhat as it beds in. Shut down, let it cool, and recheck tension after the first few hours of running.

A squeal that starts when a big electrical load comes on (refrigeration, an inverter) usually means the belt is slipping under load. BoatUS says to rule out an unusual electrical load or short first, then tighten or replace the belt.

Keep the exact belt (or a set, if your engine has more than one) aboard, plus the tensioner tool or wrenches needed to fit it. BoatUS's belt article says to always keep spare belts handy. Practice a belt change at the dock once so you know you can do it on the water.

Signs: a sudden overheat alarm or rising temperature, a battery or charging warning, a squeal followed by silence, or steering that suddenly goes heavy. Slow down and shut down the affected engine as soon as it is safe, because running without the water pump can overheat it quickly (see the overheating guide). On twin engines, you can usually come home slowly on the other one. Steering may need much more effort without power assist. If you cannot fit the spare safely, anchor if needed and call for a tow.

After a belt failure, a new belt is not always the end of it. If the engine ran hot, or the belt also drives the seawater pump, have the raw water impeller checked (it can be damaged running dry or hot) and watch the temperature closely on the next runs. An engine that overheated badly, or now runs weak or uses coolant, needs a mechanic's check before the next trip; owners in forums regularly report "lack of power" after running on with a broken belt.